1. Field of the Invention
The present invention relates to a method for producing a semiconductor device, and more specifically, to a method for forming a refractory metal silicide film on a silicon diffusion layer or a polysilicon gate electrode in a self-aligned manner.
2. Description of the Prior Art
The degree of integration of semiconductor devices is being continuously increased, and accordingly, the width of gate wiring (length of the gate electrode) and the width and thickness of the diffusion layer is being decreased. Decrease in the width of gate wiring and layer thickness inevitably results in increase in wiring resistance, causing the delay of circuit operations. In submicron semiconductor devices, the reduction of the resistance of refractory metal silicide is an important and essential technology.
In MOS semiconductor devices, the self aligned silicide technique using titanium (Ti) as a refractory metal is being successfully applied to actual devices.
In this kind of structure of transistors, however, the diffusion layer is being forced to become a shallow junction due to the trend towards increased integration. Thus, a reduced thickness of the silicide film is being demanded. The reduction of the silicide film thickness causes not only increase in sheet resistance due to the reduction of the film thickness itself, but also the following two problems on the formation of the silicide film: Firstly, the problem of the breaking (electrical disconnection) of or increase in resistance of the silicide due to agglomeration, and secondly, the problem of rise in phase transition temperature.
The cause of the breaking of the titanium silicide layer due to agglomeration is considered as follows. Titanium silicide films begin to soften at a temperature of 800.degree. C. or above, and becomes flowable. This flow occurs in the horizontal direction on the surface of the titanium silicide or at the interface with the diffusion layer. By this flow, the titanium silicide film deforms to minimize its surface energy. That is, the film separates into a thick island region and a thin film region. Due to this phenomenon, the uniformity of the titanium silicide film thickness is lost, and in some cases, the film is completely broken, i.e. disconnected electrically. The conductivity of the titanium silicide is therefore deteriorated, and resistance increases.
Furthermore, decrease in silicide film thickness and decrease in silicide wiring width both cause increase in surface tension. This results in decrease in the temperature at which agglomeration begins. This was reported by T. P. Nolan, et al. in Journal of Applied Physics, 71 (2), p. 720 (1992).
On the other hand, phase transition is required for forming a titanium silicide film. This is the phase transition from the C49 structure, which is a non-equilibrium phase having a high sheet resistance (resistivity: 2.times.10.sup.-4 .OMEGA.cm), to the C54 structure having a low sheet resistance (resistivity: 1.5.times.10.sup.-5 .OMEGA.cm). In order to reduce the size of transistors, increase in impurity concentration, decrease in the silicide film thickness, and decrease in the silicide film width are required. The phase transition temperature varies depending on these impurity concentration, silicide film thickness and silicide film width. By reducing the silicide film thickness from 50 nm to 30 nm, for example, the phase transition temperature from the C49 structure to the C54 structure increases from about 700.degree. C. to 900.degree. C. or above.
Conventional silicides are therefore formed only in the temperature range limited by phase transition temperature and agglomeration temperature, i.e., above the phase transition temperature and below the agglomeration temperature. Because of the rise in phase transition temperature and decrease in agglomeration temperature due to the above reduction of the device size, the temperature range in which suicides are formed satisfactorily is disappearing, and in the design of a device of 0.5 .mu.m or below, a film of low resistivity (10 .OMEGA./.quadrature.) can no longer be obtained by conventional silicide formation methods.
The prior art method for producing a transistor having a conventional structure will be described referring to FIGS. 1 and 2. FIG. 1(a)-FIG. 1(d) and FIG. 2(a)-FIG. 2(c) are sectional views illustrating the steps of the first embodiment of the conventional method for producing this type of transistors.
First, as shown in FIG. 1(a), on a silicon substrate 1, a gate electrode 4 consisting of polysilicon is formed on a gate oxide film 3; a part of a diffusion layer 2 is formed using the gate electrode 4 as a mask, a side wall 5 adjacent to the gate electrode is formed, and then a part of a diffusion layer is further formed, to form the diffusion layer 2 of a Lightly Doped Drain (LDD) structure in the surface region of the silicon substrate 1. Thereafter, a titanium film 6 of a desired film thickness (10-100 nm) is formed on the entire surface by sputtering.
Next, heat treatment for forming the silicide is performed for 30-60 seconds in a nitrogen atmosphere at 600.degree.-700.degree. C. For this heat treatment, a heat treatment chamber equipped with a lamp annealing device is usually used. By this, as shown in FIG. 1(b), a titanium silicide (TiSi.sub.2) film 7 of the C49 structure is formed in a region where the titanium film 6 contacts with the silicon surface, that is, the region on the surface of the gate electrode 4 and the surface of the diffusion layer 2 not covered by the insulation film. The titanium film 6 on the silicon oxide film (e.g. on the side wall 5) and the titanium film not silicified react with nitrogen to form a titanium nitride (TiN) film 9.
After the heat treatment, the wafer is removed from the heat treatment chamber. The removal is performed at a temperature of 400.degree.-500.degree. C. at which silicon is not oxidized. At this time, the titanium nitride film 9 absorbs approximately 30 percent oxygen, which is the dissolution limit of oxygen in TiN, and becomes partly oxidized. The oxygen which has passed through the partially oxidized titanium nitride film reaches the surface of the titanium silicide film 7 and oxidizes a part of the surface to form an oxygen excess silicide film 11 of a thickness of about 15 nm at the interface between the titanium silicide film 7 and the titanium nitride film 9.
Next, as shown in FIG. 1(c), the titanium nitride film 9 and the titanium film which has not reacted are removed using a mixed solution of ammonia water and hydrogen peroxide. By the above steps, a self-aligned titanium silicide film 7 is formed on the gate electrode 4 and the diffusion layer 2 forming the source and drain regions.
Thereafter, as shown in FIG. 1(d), a second heat treatment for phase transition is performed in a nitrogen atmosphere at about 800.degree. C. for about 60 seconds to form a titanium silicide (TiSi.sub.2) film 8 having the C54 structure, the resistivity of which is lower than the resistivity of the C49 structure. Next, as shown in FIG. 2(a), an interlayer insulation layer 12 consisting of SiO.sub.2 is formed using the ordinary chemical vapor deposition (CVD) process. Here, heat treatment is performed at about 600.degree. C., which is the vitrification temperature of SiO.sub.2, to remove moisture in the interlayer insulation film, and to allow the interlayer film to reflow thereby improving flatness. Next, as shown in FIG. 2(b), the interlayer insulating film 12 is selectively removed using the normally used lithography and etching to form a contact hole 13 reaching the diffusion layer 2. Finally, as shown in FIG. 2(c), the deposition and patterning of aluminum are performed to form the electrode/wiring 14, thus producing a desired semiconductor device.
Furthermore, as the second embodiment of the prior art method for producing this type of transistors, the method disclosed in JPUPA 1-205446 and IEEE International Electron Device Meeting, p. 906 (1993) will be described referring to FIG. 3. In this second embodiment also, like the first embodiment shown in FIG. 1(a), on a silicon substrate 1, a gate oxide film 3, a gate electrode 4 consisting of polysilicon and adjacent side walls 5 are formed. A diffusion layer 2 of the LDD structure is formed in the surface region of the silicon substrate 1. Next, a titanium film 6 of a desired thickness (10-100 nm) is formed on the entire surface by sputtering. Then a titanium nitride film 9a is formed by nitrogen atmosphere heat treatment, after which the water is taken out from the heat treatment chamber into atmosphere. For the following steps, the same steps as in the first embodiment shown in FIG. 1(b) and later, are performed to form titanium silicide on the gate electrode 4 and the diffusion layer 2.